Contact lenses having a toric optical zone (commonly referred to as “toric contact lenses”) are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for the astigmatism. Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. The toric surface may be formed in either the posterior lens surface (back surface toric lens) or in the anterior lens surface (front surface toric lens).
Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have some type of ballast to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism. Toric contact lenses are manufactured with a selected relationship (or offset) between the cylindrical axis of the toric optical zone and the orientation of the ballast. This relationship is expressed as the number of degrees (rotational angle) that the cylindrical axis is offset from the orientation axis of the ballast. Accordingly, toric contact lens prescriptions specify this offset, with toric lenses generally being offered in 5 or 10-degree increments ranging from 0° to 180°.
In summary, a prescription for a toric contact lens will typically specify spherical correction (spherical power), cylindrical correction (cylindrical power) and axes offset to define the optical correction, as well as lens diameter and base curve to define fitting parameters.
One type of ballast is prism ballast, which has proven effective for maintaining a toric contact lens in a desired rotational orientation on the eye. An example of prism ballasting is disclosed in U.S. Pat. No. 6,113,236. Prism may be attained by various manners including: decentering the optic zone of the lens vertically downwards, such that a “wedge” of thickness across the optic zone is achieved; or tilting the entire anterior surface with respect to the posterior surface, such that a “wedge” of thickness across the entire lens is achieved. Then, for either of these approaches, the lens periphery is designed to achieve better fitting and on-eye comfort. However, both of these techniques for introducing prism place limitations on the ability to control the peripheral region of the lens. The two primary levers for changing the peripheral thickness become the amount of decentration or angle of tilt put into the lens, and the lens center thickness (CT) to increase or decrease the overall thickness profile of the lens design. In addition, decentering the lens optic introduces the additional optical higher order aberration coma, which is an undesirable lens feature.